The goal of this research is to describe the morphological changes and to define the temporal sequences of the changes that occur in the migration and differentiation of vertebrate neurons. In this study, the primary and secondary sensory neurons of the auditory system will be studied with light and electron microscopic techniques in chick embryos so as to define the structural relationships between the auditory nerve fibers, their synaptic endings, and the hair cell receptors of the cochlear duct, on the one hand, and the second-order neurons of the cochlear nucleus, on the other hand, at each stage in the development of the auditory pathways. The long-term goal is to analyze the interactions between the peripheral sensory end-organ and the CNS in the early development of the sensory systems and to gain some insight into the mechanisms responsible for the formation of sensory neurons and their synaptic connections. The effects of end-organ ablation, and transplantation experiments will be used to test our hypotheses. This research will establish a solid morphological and experimental basis on which to pursue subsequent biochemical and cytochemical studies on the mechanisms underlying the migration and differentiation of sensory neurons and information of synapses in sensory neurons and formation on synapses in sensory systems. It will clarify the roles of sense organs in the development of the brain, and vice versa.